1. Field of the Invention
The invention relates to blackbody simulators, and more particularly to such simulators having a core with an aperture to an interior cavity, the aperture simulating the properties of a similarly sized and shaped blackbody.
2. Description of the Prior Art
A blackbody is an idealized object which would absorb all electromagnetic radiation impacting it. Since such an object would absorb all light striking it, it would appear black. The physical properties of blackbodies have been intensively studied. Although the definition of a blackbody is in terms of its perfect absorption of electromagnetic radiation, its most interesting properties are associated with its radiation energy. When considered as a radiation source, it is generally considered to be heated to increase its radiated energy. The total emission of radiant energy from a blackbody is expressed by the Stefan-Boltzmann law, which states that the total electromagnetic emission of a blackbody is proportional to the fourth power of its absolute temperature. The spectral energy distribution of the radiant energy emitted by a blackbody is expressed by Planck's radiation formula. Planck's radiation formula indicates that a blackbody which has a temperature between 50 degrees Kelvin and 3,000 degrees Kelvin will emit electromagnetic radiation principally in the infrared region. This temperature range encompasses the temperatures at which most nonnuclear physical phenomena occur.
A blackbody is an idealized concept. A blackbody simulator is an apparatus designed to simulate the physical properties of the idealized blackbody. A blackbody simulator is of great use in infrared research and development and manufacturing. For instance, it may be used to provide a source of infrared radiation of a known signal level and a known spectral distribution. It may be used to provide a source of infrared radiation for the adjustment or testing of infrared components, assemblies or systems. Another use for blackbody simulators is as a near perfect absorber of infrared radiation.
Intuitively, it would appear that a blackbody simulator would be merely any black object. Such simulators have been used in the past, but their correspondence to a true blackbody has been poor. The best blackbody simulators are formed by creating a cavity in a core material, the cavity forming an aperture on a side of the core. The aperture is used to simulate a flat blackbody having the shape and size of the aperture. Particular cavity shapes are chosen to cause multiple reflection and eventual absorption of any electromagnetic energy entering the aperture.
One measure of the closeness by which a blackbody simulator approaches a true blackbody is its emissivity. The emissivity of an isothermal surface is the ratio of the radiation emitted by the surface to the radiation emitted under identical conditions by a blackbody having the same shaped surface and temperature. Unless the blackbody simulator is luminescent, the emissivity of an isothermal blackbody simulator is less than one.
If the cavity surface coating of a blackbody simulator has an emissivity above 0.7, most cavity shapes in commercial use in blackbody simulators will result in a device in which the on-axis emissivity of the aperture will exceed 0.99.
The angular distribution of radiation from a perfectly diffuse radiator, such as a blackbody, is given by the Lambert cosine law. This law states that the signal level of radiation from a perfectly diffuse surface is proportional to the cosine of the angle between the direction of emission and a normal to the surface. Accordingly, the Lambert cosine law provides a standard against which the uniformity of emission from a blackbody simulator can be compared against a blackbody.
It is well known how to manufacture a blackbody simulator with a cavity shape configured to have an on-axis emissivity very close to one. There are commercially available blackbody simulators with an on-axis emissivity of 0.9997. Unfortunately, the prior art cavity shapes for high emissivity blackbody simulators have been found to not provide the uniformity of emission specified by the Lambert cosine law. Such uniformity of emissivity is becoming a significantly more important consideration in current infrared research and development. It appears that the uniformity of blackbody simulator emissivity will become a significantly more important consideration in future manufacturing adjustments and tests of infrared components, assemblies and systems.
For instance, infrared viewing systems have been the subject of intensive research and development. Initially, a single element infrared sensor was used with a two axis mechanical scanner to provide a two dimensional infrared picture. For calibration and testing of such single elements, a blackbody simulator was used to illuminate a relatively narrow field of view. Since a single element infrared sensor was used, uniformity of emissivity was not as important as it is today. However, infrared viewing systems now in development use a one dimensional or two dimensional array of infrared sensors, thereby eliminating the need for one direction or both directions of mechanical scanning. Testing and calibration of such array detector systems require a blackbody simulator which has an emissivity which is uniform over the appropriate angular field of view.
Although emissivity and uniformity of emissivity are important specifications for a blackbody simulator, such simulators also are developed with other very practical considerations in mind.
Key specifications for a blackbody simulator to be used as a source of infrared emission include aperture size and temperature range. The cost of a blackbody simulator is in large part determined by the physical size and weight of the blackbody simulator. The cost is also affected by the particular cavity shape used inasmuch as certain cavity shapes are more expensive to manufacture.
The radiation properties of a cavity type blackbody simulator are determined by the size and shape of the cavity and the temperature, material, and texture of the cavity walls. Most commercially available blackbody simulators have a cavity shape based upon a cone. Other popular shapes are the sphere, the reentrant cone and the cylinder. A reentrant cone cavity has a shape which is formed by placing base to base two circular cones having the same size base, and truncating the apex of one of the cones to form the aperture. Both a conical cavity and reentrant cone cavity have a conical apex opposite the aperture.
An advantage to a blackbody simulator having a spherical cavity is that its emissivity is very uniform. Further, it has been theoretically proved and experimentially verified that the surface of a spherical cavity tends to become isothermal, i.e., the cavity surface temperature tends to become uniform. It is desirable for the surface of the cavity to be as nearly as possible isothermal since that property is necessary for the spectral distribution of the simulator to conform to the Planck radiation formula. Despite these advantages, a spherical blackbody simulator is a large object, and is correspondingly very heavy. In addition, its on-axis emissivity can be achieved by consideredly smaller blackbody simulators having conical or reentrant cone cavities. Another disadvantage to a blackbody simulator using a spherical cavity is specular reflection from the cavity wall opposite the aperture. Radiation entering the aperture on-axis tends to be reflected out the aperture, rather than be absorbed. Such specular reflections are antithetical to the definition of a blackbody. This latter deficiency of a spherical cavity blackbody simulator is sometimes overcome by using a tilted or off center arrangement for the sphere and the core, but this technique requires even larger and heavier assemblies.
Blackbody simulators having a generally cylindrical cavity shape, with the aperture in one axial end of the cylinder, are easy to manufacture. Unfortunately, they have lower on-axis emissivity than a cone, and they have poor uniformity of temperature and poor uniformity of emissivity.
Most commercial blackbody simulators have a cavity shape which is either a cone or reentrant cone. Such shapes have excellent on-axis emissivity. Specifically, the apex of the cone opposite the aperture has emissivity much greater than that of a sphere with a diameter such that the wall of the sphere opposite the aperture would be at the same distance from the aperture as that of the apex of a cone or a reentrant cone. A blackbody simulator having a cone as a cavity shape is relatively easy to manufacture, except for the apex, but it has poor uniformity of emission. The reentrant cone is more difficult to manufacture, but the advantage of a reentrant cone over a simple cone is that the surface of the cavity maintains a more uniform temperature and emissivity than that of a similar sized cone, since a reentrant cone cavity tends to minimize the cooling of the cavity near the aperture.
It is an object of the invention to provide a cavity type blackbody simulator that has the high on-axis emissivity obtained with cone or reentrant cone blackbody simulators, yet also approaching the uniform emissivity obtained with spherical blackbody simulators. Another object of the invention is to provide a compact blackbody simulator which is shorter and smaller in diameter than a blackbody simulator having a spherical cavity with the same emissivity. Yet another object of the invention is to provide a blackbody simulator with a cavity having a shape which tends to take a uniform temperature when heated. A further object of the invention is to provide a method for the design of a blackbody simulator cavity having a specified maximum depth, diameter, and aperture size in which high emissivity and uniformity of emissivity are obtained.